Inkjet printing is well suited to the SOHO (small office, home office) printer market. Each printed pixel is derived from one or more ink nozzles on a printhead. This form of printing is inexpensive, versatile and hence increasingly popular. The ejection of ink can be continuous (see U.S. Pat. No. 3,596,275 by Sweet) or the more predominant ‘drop-on-demand’ type in which each nozzle ejects a drop of ink as it passes across a media substrate location requiring a drop of ink. Drop on demand printheads typically have an actuator corresponding to each nozzle for ejecting ink.
The actuators can be piezoelectric such as that disclosed by Kyser et al in U.S. Pat. No. 3,946,398. However, recently electro-thermally actuated printheads have become most prevalent in the field of inkjet printing. Electro-thermal actuators are favored by manufacturers such as Canon and Hewlett Packard. Vaught et al in U.S. Pat. No. 4,490,728 discloses the basic operation of this type of actuator within an inkjet printhead.
Wide format printing is another market in which inkjet use is expanding. ‘Wide format’ can refer to any printer with a print width greater than 17″ (438.1 mm). However, most commercially available wide format printers have print widths in the range 36″ (914 mm) to 54″ (1372 mm). Unfortunately, wide format printers are excessively slow as the printhead prints in a series of transverse swathes across the page. To overcome this, there have been attempts to design printers that can print the entire width of the page simultaneously. Examples of known pagewidth thermal inkjet printers are described in U.S. Pat. No. 5,218,754 to Rangappan and U.S. Pat. No. 5,367,326 to Pond et al. A pagewidth printhead does not traverse back and forth across the page and thereby significantly increases printing speeds. However, proposals for a pagewidth printhead assembly have not become commercially successful because of the functional limitations imposed by standard printhead technology. A 600 dpi thermal bubble jet printhead configured to extend the entire width of a 1372 mm (54 inch) wide standard roll of paper would require 136,000 inkjet nozzles and would generate 24 kilowatts of heat during operation. This is roughly equivalent to the heat produced by 24 domestic bar heaters and would need to be actively cooled using a heat exchange system such as forced air or water cooling. This is impractical for most domestic and commercial environments, as the cooling system for the printer would probably require some type of external venting. Without external venting, the room housing the printer is likely to over heat.
As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes. These include inexpensive construction and operation, high speed operation, safe and continuous long term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables. Some of the perennial problems and ongoing design imperatives are addressed or ameliorated by aspects of the present invention. These design issues are discussed below.
1. Media Feed
Most inkjet printers have a scanning printhead that reciprocates across the printing width as the media incrementally advances along the media feed path. This allows a compact and low cost printer arrangement. However, scanning printhead based printing systems are mechanically complex and slow to maintain accurate control of the scanning motion. Time delays are also due to the incremental stopping and starting of the media with each scan. Pagewidth printheads resolve this issue by providing a fixed printhead spanning the media. Such printers are high performance but the large array of inkjet nozzles is difficult to maintain. For example wiping, capping and blotting become exceptionally difficult when the array of nozzle is as long as the media is wide. The maintenance stations typically need to be located offset from the printheads. This adds size to the printer and the complexity of translating the printheads or servicing elements in order to perform printhead maintenance. There is a need to have a page wide solution that is simpler and more compact.
2. Media Feed Encoder
Similarly, precise control of media feed is essential for print quality. The advance of media sheets past the printhead is traditionally achieved with spike wheel and roller pairs in the media feed path. Typically a spike wheel and roller monitors a sheet upstream of the printhead and another spike wheel and roller is downstream of the printhead so that the trailing edge of the sheet is printed correctly. These spike wheels can not be incorporated into any drive rollers and so add considerable bulk to the printing mechanism.
3. Printer Operation
The gap between the ink ejection nozzles and the media surface needs to remain constant in order to maintain print quantity. Precise control of media sheets as they pass the printhead is crucial. Any media buckling or lack of positional control of the leading or trailing edges within the print zone can result in visible artifacts.
4. Service Modules
Maintaining printheads (i.e. routine wiping, capping and blotting etc) requires maintenance stations that add bulk and complexity to printers. For example, scanning printhead service modules are typically located to one side of the media feed path and laterally offset from the printheads. This adds lateral size to the printer and the complexity of translating the printheads to the service modules in order to perform maintenance. Often the printheads move to these service modules when not printing. When each printhead returns to its operative position, its alignment with the other printheads is prone to drift until eventually visible artifacts demand realignment of all the printheads. In other cases, the service modules translate from the sides to service the printheads while the printheads are raised sufficiently above the media. Both of these system designs suffer from drawbacks of large printer width dimensions, complicated design and control, and difficulty in maintaining printhead alignment.
5. Aerosol Removal
Aerosol generation refers to the unintentional generation of ink drops that are small enough to be air borne particulates. Aerosols increase as the system speed and resolution increases. As the resolution increases, the drop volumes are reduced and more prone to becoming aerosol. As the system speed increases, velocity of the media increase, drop production rate increases and hence aerosols also increase.
The solution to this problem has been aerosol collection systems. The design of these systems becomes more challenging when the printing system utilizes a fixed printhead assembly spanning a media path that allows the use of varying media widths. When the media width is less than the full paper path width, only part of the printhead assembly operates. Portions of the printhead assembly that extend beyond the media can clog as water in the nozzles evaporate and the localized ink viscosity increases. Eventually the viscosity at the nozzle is too much for the ejection actuator to eject. Thus there is a problem of aerosol generation and the related problem of a need to exercise drop generators across and beyond the media. These problems have not been properly addressed. Prior solutions include: (1) aerosol collection system ducts that typically collect aerosol from a single duct; (2) spittoons that are placed out of the print zone that are only utilized when the printer is not printing—to name two examples.
6. Ink Delivery
Larger printheads help to increase print speeds regardless of whether the printhead is a traditional scanning type or a pagewidth printhead. However, larger printheads require a higher ink supply flow rate and the pressure drop in the ink from the ink inlet on the printhead to nozzles remote from the inlet can change the drop ejection characteristics.
Large supply flow rates necessitate large ink tanks which exhibit a large pressure drop when the ink level is low compared to the hydrostatic pressure generated when the ink tank is full. Individual pressure regulators integrated into each printhead is unwieldy and expensive for multicolor printheads, particularly those carrying four or more inks. A system with five inks and five printheads would require 25 regulators. Moreover long printheads tend to have large pressure drops with a single regulated source of ink. A multitude of smaller ink supply tanks creates a high replacement rate which is disruptive to the operation of the printer.
7. Priming/De-priming and Air Bubble Removal
Inkjet printers that can prime, de-prime and purge air bubbles from the printhead offer the user distinct advantages. Removing an old printhead can cause inadvertent spillage of residual ink if it has not been de-primed before decoupling from the printer. Of course, a newly installed printhead needs to be primed but this occurs more quickly if the printer actively primes the printhead rather than a passive system that uses capillary action.
Active priming tends to waste a lot of ink as the nozzles are fired into a spittoon until ink is drawn to the entire nozzle array. Forcing ink to the nozzles under pressure is prone to flood the nozzle face. Ink floods must be rectified by an additional wiping operation before printing can commence.
When the printhead is going to be inactive for an extended time, it can be beneficial to de-prime it during this standby period. De-priming will avoid clogging from dried ink in the nozzles and tiny ejection chambers. De-priming for standby necessitates an active and timely re-priming when next the printer is used.
Air bubbles trapped in printheads are a perennial problem and a common cause of print artifacts. Actively and rapidly removing air bubbles from the printhead allows the user to rectify print problems without replacing the printhead. Active priming, de-priming and air purging typically use a lot of ink particularly if the ink is drawn through the nozzles by a vacuum in the printhead capper. This is exacerbated by large arrays of nozzles because more ink is lost as the number of nozzles increases.
8. Carrier Assembly
Controlling the gap between the nozzles and the surface of the print media is crucial to print quality. Variation in this ‘printing gap’ as it is known affects the ink droplet flight time. As the nozzles and the media substrate move relative to each other, varying the flight time of the droplets shifts the position printed dot on the media surface.
Increasing the size of the nozzle array, or providing several different nozzle arrays will increase print speeds. However, larger nozzle arrays and multiple separate nozzle arrays greatly increase the difficulty to maintain a constant printing gap. Typically, there is a compromise between the production costs associated with fine equipment tolerances, and print quality and or print speed.
9. Ink Conduit Routing
The ink supply to all the nozzles in a nozzle array should be uniform in terms of ink pressure and refill flow rate. Changing these characteristics in the ink supply can alter the drop ejection characteristics of the nozzle. This, of course, can lead to visible artifacts in the print.
Larger nozzle arrays are beneficial in terms of print speed but problematic in terms of ink supply. Nozzles that are relatively remote from the ink feed conduit can be starved of ink because of the consumption of ink by more proximate nozzles.
At a more general level, ink feed lines from the cartridge or other supply tank, to the printhead should be as short as possible. Printhead priming operations need to be configured to the ink color with the longest flow path from the ink reservoir. This means the nozzles in the array fed by other ink reservoirs may prime for longer than needed. This can lead to nozzle floods and wasted ink.